The crankshaft is normally used when installed in an engine. When the rotational imbalance occurs in the crankshaft, a variety of troubles occur (e.g., generation of vibration in the engine revolution). Therefore, the amount of the rotational imbalance (hereinafter simply referred to as “an imbalance value”) of a crankshaft is required to be in a predetermined allowable value range. A position of a center hole, used as a reference for working the crankshaft, is important for setting the imbalance value of the crankshaft to be in the allowable value range.
For example, a center hole is formed as follows. First, a balance center axis of a crankshaft of a material state (i.e., a material crankshaft) is found. Specifically, a balance meter measures the balance of the material while the material crankshaft is actually revolved. Then, center holes are bored in both end surfaces of the crankshaft, on the obtained center axis. Further, the balance meter measures balance about the center axis again in the final working phase. When imbalance is found, the balance adjustment is executed by boring a hole in a counterweight.
When the crankshaft in a material state (i.e., the material crankshaft) is finished in an ideal shape, matching the design data, it is possible to easily achieve the imbalance value to be in the allowable value range by boring a center hole in the center of a main journal. Even if balance disruption occurs in the subsequent working steps, balance adjustment can be easily executed by boring a hole in the counterweight at the end.
However, thickness of the material crankshaft could be formed unevenly due to a variety of factors including e.g., failure of casting molds, failure of forging molds, impact in trimming and mold stripping. Most of the steps of working the material crankshaft are herein executed for a cylindrical part of a main journal and a cylindrical part of a pin journal. Therefore, the drawback of uneven thickness is resolved for the respective cylindrical parts. However, the thickness of the counterweight remains to be uneven because working is hardly executed for the counterweight. Consequently, the imbalance remains in the entire material crankshaft.
Imbalance accordingly exists in the entire crankshaft even when a hole is bored in the counterweight in the final balance adjustment. Further, when the balance adjustment cannot be completed within a predetermined cycle time, such a crankshaft is regarded as an inferior product. In this case, there is a limitation for the number of holes allowed to be bored in the balance adjustment. Therefore, an imbalance value within the allowable value range cannot be achieved in some cases.
Further, an essential portion could be required to be removed from the crankshaft in order to achieve the balance value within the allowable value range.
When the imbalance value of the crankshaft cannot be finally achieved within the allowable value range as described above, modification is required for treatment in the center hole boring processing. Specifically, the misalignment amount of the center hole boring position is computed based on the balance measurement. The misalignment amount is then fed back to the center hole boring processing. Treatment in the center hole boring processing is modified based on the feedback. When a center hole is herein bored in a material crankshaft before the feedback of the misalignment amount, the material crankshaft is regarded as an inferior product due to misalignment of the center hole. Additionally, feedback of the misalignment amount is required every time a product lot of the material is changed. In this case, there is a drawback that a great number of processing steps are required.
Japan Laid-open Patent Application Publication No. JP-A-H09-174382 describes a method as a technique for determining a position of a center hole in a crankshaft. In the method, sampling is performed for a plurality of materials, and statistic processing is executed for factors (e.g., a difference between an axial center in working of a center hole and a position of a bored center hole) affecting the imbalance amount after execution of a finishing work. Accordingly, the correction amount is obtained for determining the position of a center hole.
Further, Japan Laid-open Patent Application Publication No. JP-A-S51-076682 describes a technique for determining a position of a center hole in a crankshaft. In the method, a dynamic balance point is obtained for the respective end surfaces of a material crankshaft through a dynamic balance test. Subsequently, shapes of a journal section and the like in the material crankshaft are measured. Imbalance, possibly occurring after working, is arithmetically computed based on the result of the measurement. A center hole is then bored in a corrected position displaced from the dynamic balance point by the imbalance amount.